Dialectric grease would not hurt anything on the back of an ignition switch, but since it isn't really exposed to the "elements" I doubt it would help except perhaps possible corrosion. Given the voltages involved, the only wire where this would be a major problem is the main power wire on the battery side. Antiseize on the spark plug threads also won't hurt anything. Do your plugs have the extra copper washer on them? If so, this is a compression ring and proper torque would be best without antiseize and even if you don't have the copper washer, the biggest problem is with "like" materials (e.g. aluminum heads, and aluminum-based spark plugs). If you have that combination I would use antiseize.